As described in US Patent Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electric latent image by evenly charging a photoconductive insulating layer and subsequently exposing the layer to eliminate the charge in the exposed portion and visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As stated above, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, thereby deteriorating after copying from several thousand to several tens of thousand sheets. Such deterioration of the toner can be prevented by using a tough resin having such a high molecular weight as to withstand the above mechanical friction. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method such as oven fixing or radiant fixing with infrared rays, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method such as a heat-and-pressure fixing method using a heat roller, etc., which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device, curling of paper and an increase in energy consumption. Furthermore, the resin described above is poor in grindability, thereby remarkably lowering the production efficiency of the toner upon the production of the toner. Accordingly, the binding resin having too high a degree of polymerization and a softening point cannot be used therefor.
Meanwhile, according to the heat-and-pressure fixing method using a heat roller, etc., the surface of a heat roller contacts the surface of a visible image formed on an image-receiving sheet under pressure, so that the thermal efficiency is excellent and therefore widely used in various copying machines from those of high-speed ones to those of low-speed ones. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause a so-called off-set or offset phenomenon, wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the heat roller is treated with a material excellent in release properties, such as a fluororesin, and further a releasing agent such as silicone oil is applied thereon. However, the method of applying a silicone oil, etc. necessitates a larger-scale fixing device, which is not only expensive but also complicated, which in turn may undesirably become causative of various problems.
Although processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been disclosed in Japanese Patent Examined Publication No. 493/1982 and Japanese Patent Laid-Open Nos. 44836/1975 and 37353/1982, the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature of causing high-temperature offsetting as much as possible, the service fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of the curling of paper.
From the above reasons, the development of a toner excellent in fixing ability and offset resistance has always been expected.
There has been proposed a method for achieving improvement on the low-temperature fixing ability by using a toner comprising a core material and a shell formed thereon so as to cover the surface of the core material.
Among such toners, those having a core material made of a low-melting wax which is easily plastically deformable, as described in U.S. Pat. No. 3,269,626, Japanese Patent Examined Publication Nos. 15876/1971 and 9880/1969, and Japanese Patent Laid-Open Nos. 75032/1973 and 75033/1973, are poor in fixing strength and therefore can be used only in limited fields, although they can be fixed only by pressure.
Further, with respect to toners having a liquid core material, when the strength of the shell is low, the toners tend to break in the developing device and stain the inside thereof, even though they can be fixed only by pressure. On the other hand, when the strength of the shell is high, a higher pressure is necessitated in order to break the capsule, thereby giving too glossy images. Thus, it has been difficult to control the strength of the shell.
Further, there has been proposed, as a toner for heat-and-pressure fixing, a toner of a microcapsule type for heat roller fixing which comprises a core material made of a resin having a low glass transition temperature which serves to enhance the fixing strength, though blocking at a high temperature may take place if used alone, and a shell made of a high-melting point resin which forms a wall by interfacial polymerization for the purpose of imparting blocking resistance, etc. of the toner (see Japanese Patent Laid-Open No. 56352/1986). However, this toner cannot fully exhibit the performance of the core material, because the melting point of the shell material is too high. Further, it has been difficult to freely control the chargeability of the shell formed by interfacial polymerization. On the same line of thinking as that described above, encapsulated toners for heat roller fixing with an improved fixing strength of the core material have been proposed (see Japanese Patent Laid-Open Nos. 128357/1988, 128358/1988, 128359/1988, 128360/1988, 128361/1988 and 128362/1988). However, since these toners are prepared by a spray drying method, a higher load to the equipments for the production thereof becomes necessary. In addition, they cannot fully exhibit the performance of the core material, because they have not come up with a solution for the problems in the shell.
Further, there have been attempts to control the chargeability of the encapsulated toner in the presence of a charge control agent in the shell of the encapsulated toner or on the surface of the encapsulated toner. However, in the developing process, since the charge control agent becomes detached from the toner due to friction with carrier, etc. and is adhered to the carrier, the electric charge of the resulting toner is lowered, thereby causing such problems as background contamination and scattering of the toner in the developer device. In addition, when no charge control agents are present on the surface of the toner, charging speed may become slow depending upon the type of carriers, thereby causing background contamination, scattering of the toner, etc. in the case of quick printing.